According to the mental health charity Mind, 17% of Britons suffer from some depression or anxiety, and the World Health Organisation estimates that by 2020 depression will become the second most important cause of disability in the world. So why are so few sufferers willing to tell friends and work colleagues about their problems?

In theory, employers should want to encourage workers to be open about their mental health. Andy Bell, of the Sainsbury Centre for Mental Health, has estimated that the cost of ignoring mental distress at work is £1,000 for every employee in UK business. Time to Change, a coalition of charities campaigning to end mental health discrimination, says it is in the interests of employees too: talking about mental health problems could help them keep their jobs. Yet the stigma remains.

"People find it very difficult to know what they are supposed to do when a friend or colleague says they have a mental health problem," said Sophie Corlett, the charity's director of external relations. "They see it as mysterious, different and requiring a specialised response but actually all people often need is a friendly face, people around them, carrying on making suggestions of things to do and being included. There is also a stigma that people with mental health problems are less capable, less interesting, less intelligent and even dangerous."

Sufferers are often aware of these potential reactions, and may even have held the same views before they became ill. On top of that, depression or anxiety means they "have very low self-esteem, feel that no one wants to hear and that they are unlikely to get better anyway", Corlett added. Equally, the lack of an obvious disability encourages sufferers to believe they can keep their condition secret.